“Dipeptides” in which two amino acids are bonded have been remarked as functional materials. The dipeptides make it possible to add a physical property not found in a single amino acid, or add a new function, so that the dipeptides have been expected to have a wider range of applications beyond the amino acids. Among them, diketopiperazines, which are cyclic dipeptides, have been known to have various physiological activities, and it has been expected that the diketopiperazines will expand in demands in the fields of medicine and pharmacology.
For example, Patent Publication 1 has reported that cyclic dipeptides having 2,5-diketopiperazine structures have anti-depressant action, action for improving learning motivation or the like. Also, Non-Patent Publication 1 describes that a cyclic dipeptide Cyclo(His-Pro) shows diversified physiological activities, including actions involving the central nervous system, such as lowering of body temperature and suppression of appetite; and hormone-like actions such as inhibition of prolactin secretion and promotion of growth hormone secretion, and the publication also reports that a cyclic dipeptide Cyclo(Leu-Gly) shows actions for improving memory functions, and that a cyclic dipeptide Cyclo(Asp-Pro) shows an action of inhibiting fat preference. Non-Patent Publication 2 has reported cyclic dipeptides having an anti-bacterial action and an anti-oxidative action.
In addition, Non-Patent Publication 3 describes that a cyclic dipeptide Cyclo(Trp-Pro) has an anti-cancer action, that cyclic dipeptides Cyclo(His-Pro) and Cyclo(Gly-Pro) have an anti-bacterial action, that a cyclic dipeptide Cyclo(His-Pro) has a neuroprotectant action, that a cyclic dipeptide Cyclo(Gly-Pro) has an action for improving memory functions, and that cyclic dipeptides Cyclo(Tyr-Pro) and Cyclo(Phe-Pro) have an action as a biological herbicide.